Electromagnetic voltage generator



Oct. 18, 1960 w. w. BUECHNER ELECTROMAGNETIC VOLTAGE GENERATOR 2Sheets-Sheet 1 Filed Oct. 21, 1957 figs I A+B Oct. 18, 1960 w. w.BUECHNER 2,957,094

ELECTROMAGNETIC VOLTAGE GENERATOR Filed Oct. 21, 1957 2 Sheets-Sheet 2United States Patent 2,957,094 ELECTROMAGNETIC VOLTAGE GENERATOR WilliamW. Buechner, Arlington, Mass., assignor to High Voltage EngineeringCorporation, Burlington, Mass, a corporation of Massachusetts.

Filed Oct. 21, 1957, Ser. No. 696,008,

12 Claims. or. 310-11 1 This invention relates to electromagneticapparatus for the generation or high voltage for acceleratingsubstantially monoenergetic beams of' charged particles. There is a needfor monoenergetic beams of charged particles in many fields, includingradiation chemistry, electron sterilization, and nuclear physics. Wherea source of ionizing radiation takes the form of a beam of high-energyelectrons, it is particularly desirable that the electron beam bemonoenergetic, in order to avoid undesired side effects resulting fromexcessive dosage and also in order to minimize the inefiiciency. whichresults from non-uniform dosage. Moreover, the electron beams used as asource of radiation are frequently scanned by a scanning device which ingeneral operates best upon a monoenergetic beam. Where only a limitedamount of power is required, a highly monoenergetic beam of chargedparticles can. be obtained with an electrostatic generator. However,where high powers are desired for direct-voltage acceleration of chargedparticles, it is necessary to use an electromagnetic voltage generator,and itmay be desired to use an electromagnetic voltage generator even atlower powers. Since electromagnetic devices produce high voltage throughthe medium of a changing magnetic flux, the resultant voltage output isneceesarily A .C., and the wave form is generally sinusoidal. A sinewave voltage can be used to produce an approximately monoenergetic beam,but in that event one can only make use of the peak of the wave.Moreover, the low-voltage portion of the sine wave is useless in anyevent, since the. energy of low velocity electrons would be entirelylost in the electron window. Improved operation of electromagneticdevices for accelerating monoenergetic beams of charged particles may beobtained by rectifying the AC. voltage output. This involvesconsiderable expense in a high voltage device, since many rectifiersmust be provided. Moreover, there is always the danger that one or moreof the rectifiers may brealc down. Conservation of space in high-powerdevices'would require the use of small rectifiers such as silicondiodes, which would still further increase the ex- Pen In accordancewith the invention, the electromagnetic generator is caused to producean approximately squarewave voltage output, so that the voltage isconstant dur ing the period in which charged particles are accelerated.No charged particles are accelerated when the voltage is reversed,unless a double high-voltage accelerator is used, as described in detailhereinafter. In this way the necessity for rectifiers is avoided. Sincethe output voltage of an electromagnetic generator is proportional tothe time rate of change of the magnetic flux linked by the output coilof the generator, a square-wave voltage output is produced'by causingthe magnetic flux to vary in a saw-tooth manner. If the electromagneticvoltage generator is a transformer, the desired wave form is achieved bydriving the primary with a current having sawtooth wave form. If theelectromagnetic voltage generator is an alternator, the desired voltagewave form is achieved 2,957,094 Patented Oct. 18, 1960 by appropriateshaping of the pole faces of the rotor, so that the resultant variationin the magnetic flux is of sawtooth wave form.

The invention may be used with any electromagnetic voltage source, suchas a transformer or an alternator. However, the invention isparticularly useful when used with the voltage generator described andclaimed in a co-pending application, Serial No. 647,915, filed March 22,1957, and asigned to the assignee of the present invention. That is tosay, the invention is particularly useful when used with electromagneticapparatus wherein the magnetic circuit includes a core havingelectrically insulating, properties along the lines of magnetic flux.For example, such a core may comprise ferromagnetic material having atleast two gaps each crossing a substantial portion of the lines ofmagnetic flux, and together insulating a potential difference in excessof 10 kilovolts, the ratio of the length of each gap to the area thereofbeing small, so as to reduce reluctance of the gap.

The invention may best be understood from the following detaileddescription thereof having reference to the accompanying'drawings inwhich:

Fig. 1 is a diagram illustrating one embodiment of the invention inwhich the electromagnetic generator is a transformer;

Fig. 2 is a graph showing the current outputs of the alternators whichdrive the primary of the transformer shown in Fig. 1;

Fig. 3 is a graph showing how a sawtooth wave form is correspondinglyapproximated as higher frequencies are added to the primary current;

Fig. 4 is a graph comparing the square wave voltage output with thesawtooth wave current input;

Fig. 5 is a diagram showing an alternator adapted to generate a squarewave voltage in accordance with the invention; and

Fig. 6 is a somewhat diagrammatic view of a charged particle acceleratorin which both half-cycles of the square-wave voltage are used toaccelerate charged particles.

Referring to the drawings and first to Fig. 1 thereof, four alternators1 are supported on a common shaft 2 which is driven by a motor 3. Thealternators 1 are adjusted so that their frequencies are related inaccordance with the Fourier series hereinafter discussed. For example,the alternators 1 may have the freqencies 60, 1 8i), 300 and 420' cyclesper second. The alternators having frequencies of 60 and 300 cycles persecond should be in phase with each other, and the alternators havingfrequencies of and 420 cycles per second should be in phase with eachother but 180 out of phase with the first pair of alternators. Since thealternators are driven by a common shaft, they will maintain their phaserelationship once it has been properly established. The current outputsof the four alternators 1 are used to drive the primary or primaries 4of a transformer 5. Any suitable arrangement for insuring that therelative powers delivered to the primary circuit 4 by the alternators 1are in accordance with the aforementioned Fourier series may beemployed. In the device shown in Fig. 1, the appropriate relative powersare achieved by providing a separate primary coil 4 for each alternator1, the number of turns in each primary 4 being adjusted to give theproper power relative to the other alternators. That is to say, thepower output of the alternators having frequency outputs of 180, 300,and 420 cycles per second should be, respectively, A,, and A of thepower output of the alternator having a frequency of 60 cycles persecond.

By combining the power outputs of several alternators, each havingdifferent frequencies and different power outputs, a sawtooth wave formmay be approximated in accordance with Fourier analysis. One Fourierseries which generates a very accurate saw tooth wave with only a fewdifiiferent frequencies, owing to the rapidity with which the amplitudedrops off at higher frequencies, is the following, the first four termsof which are" employed in the device shown in Fig. 1. 1

8 1 1 l I i [S171 sm 30+- sin 56 4g sin (Zn-D0 The current output ofeach alternator is shown in the graph of Fig. 2 and the results ofcombining the outputs of one, two, three, and four alternators are shownin the graph of Fig. 3. As shown in the graph of Fig. 4, the voltageoutput of the transformer 5, which is proportional to the firstderivative of the current in the primary 4 is of square-wave form.

If the electromagnetic generator operates as an alternator rather thanas a transformer, as shown in Fig. 5, the required square-wave voltageoutput may be achieved merely by proper shaping of the pole faces 6 ofthe rotor. It will be recalled that the generation of a sine wave by analternator is diflilcult to accomplish and is now accomplished only as aresult of much empirical investigation. Similarly, the proper shape of apole face for generating a saw-tooth Wave form cannot be determined withaccuracy theoretically and must be determined by experiment. However, itmay be said that it would be easier to develop a rotor for generating asaw tooth wave then it has been to develop a rotor for generating a sinewave. p

The principles utilized the transformer type embodinieh't of theinvention shown in Figs. 1 through 4 may also be applied to thealternator-type embodiment of the invention shown in Fig. 5. That is tosay, the rotor shown in Fig. 5 may be laminated, as it were, so as tocomprise a series of layers, each of which is a rotor havingcharacteristics different from those of the others; for example, thenumber of rotor arms might vary among layers so as to introducedififerent frequency components. 7 In the device described in connectionwith Figs. 1 through 4, there is only one secondary or voltage outputcoil 7, so that charged particles can be accelerated 'only duringalternate half-cycles. In order to get the benefit of the otherhalf-cycle, another embodiment of the invention may be used as shown inFig. 6, in which a U-shaped insulating transformer core 8 is used. Theprimary coils 9 are wound around the bridge of the U 'and a high voltageterminal 10, 11 is supported at 'e'ach end of the U 8. The two terminals10, 11 may be separated by an insulating keystone core 12 of magneticmaterial, which would improve the electrostatic as well as the magneticcondition of the apparatus. For more power, an additional primarycircuit 13 could be added about the grounded center of this keystonecore 12. At least one acceleration tube 14, 15 would be required foreach terminal 10, 11. The two tubes 14, 15 may be so positioned that thebeams 16, 17 therefrom converge on the product 18 to be irradiated, asshown in Fig. 6, or the beam from one tube could be deflected so as toproduce the same effect. The two secondaries 19, 20 corresponding to thetwo terminals 10, 11 would be wound in opposite senses, so that theterminals 10, 11 would alternately be at high negative potential and the.acceleration tubes 14, 15 would alternately deliver their electronbeams 16, 17 respectively, onto the product 18.

Having thus described the method of the invention, together with several.illustrative embodiments of the apparatus for carrying out the method,it is to be understood that, -although specific terms are employed, theyare used in a generic and descriptive sense and not for purposes oflimitation, the scope of the invention being setljfjorthiin following iI claim:

1. A method of generating a high voltage suitable for the accelerationof substantially monoenergetic charged particles which method comprisesgenerating in the same region of space a plurality of magnetic-fluxcomponents having the same or opposite orientation and each of whichvaries sinusoidally with time and has a frequency, amplitude and phaserelated to those of the other components; superimposing saidmagnetic-flux components upon each other so as to produce a magneticflux which varies with time in a sawtooth manner; and converting saidchanging magnetic flux into a high voltage which varies with time in asawtooth manner; and converting said changing magnetic flux into avoltage in excess of 500 kilovolts which varies with time in square wavemanner.

2. A method according to claim 1 wherein said frequencies, amplitudesand phases are proportional to a sequence of initial terms in thefollowing Fourier series:

quencies, amplitudes and phases are proportional to the following terms:

sin 70+ sin (2n l)0 Sin 0-; sin 394 sin 56- sin 76 4. Apparatus forgenerating high voltage suitable for the acceleration of substantiallymonoenergetic charged particles comprising in combination a magneticcore linked by a conductive path which is adapted to generate voltage inexcess of 500 kilovolts in response to changes in the magnetic flux insaid magnetic core, and means for generating a changing magnetic flux insaid magnetic core, the time variation of said magnetic flux being ofsawtooth wave form, said magnetic core including a high voltage terminaland means for insulating said high voltage terminal from the groundedparts of said magnetic core.

5. Apparatus for generating high voltage suitable for the accelerationof substantially monoenergetic charged particles comprising incombination a magnetic core linked by a conductive path which is adaptedto generate voltage in excess of 500 kilovolts in response to changes inthe magnetic flux in said magnetic core, a plurality of primary coilslinking said magnetic core, means for generating current in each of saidprimaries which current varies sinusoidally with time and has afrequency, amplitude and phase related to those of the currents in theother primaries, whereby a magnetic flux is produced in said magneticcore which varies with time in a sawtooth manner. 1

6. Apparatus according to claim 5, wherein said means for generatingcurrent comprises a plurality of alternators driven by a common shaft,each primary coil being energized by a separate alternator.

7. Apparatus for generating high voltage suitable for the accelerationof substantially monoenergetic charged particles comprising incombination a magnetic circuit including two high-voltage terminals andtwo magnetic cores having electrically insulating properties along thelines of magnetic flux, said high-voltage terminals being respectivelymagnetically connected to ground by said magnetic cores havingelectrically insulating properties along the lines of magnetic flux,said high-voltage terminals being electrically connected to ground bytwo conductive paths linking said magnetic cores having electricallyinsulating properties along the lines of magnetic flux respectively andbeing wound in opposite senses and adapted to generate high'voltage inresponse to changes in the magnetic flux in said magnetic circuit, andmeans for inducing a changing magnetic flux in said magnetic circuitwhich varies with time in a sawtooth manner.

8. Apparatus according to claim 7, wherein said magnetic circuit alsoincludes an auxiliary magnetic core having electrically insulatingproperties along the lines of magnetic flux which provides a magneticconnection between said high-voltage terminals.

9. Apparatus according to claim 8, wherein said means for inducing achanging magnetic flux in said magnetic circuit includes at least oneconductive path linking said auxiliary magnetic core having electricallyinsulating properties along the lines of magnetic flux.

10. A method of generating a high voltage suitable for the accelerationof substantially monoenergetic charged particles, which method comprisesproducing a magnetic flux which varies with time in a sawtooth manner;and converting said changing magnetic flux into a voltage in excess of500 kilovolts which alternates substantially abruptly between twosubstantially fixed potentials.

11. Apparatus for generating high voltage suitable for the accelerationof substantially monoenergetic charged particles, comprising incombination a magnetic core linked by a conductive path which is adaptedto generate voltage in excess of 500 kilovolts in response to changes inthe magnetic flux in said magnetic core, and means for producing atleast one electric current which links said magnetic core and whichvaries in time in such a manner that the resultant magnetic fluxproduced thereby in said magnetic core varies with time in a sawtoothmanner.

12. Apparatus for generating high voltage suitable for the accelerationof substantially monoenergetic charged particles comprising incombination an elongated magnetic core linked by a conductive path whichis adapted to generate voltage in excess of 500 kilovolts in response tochanges in the magnetic flux in said magnetic core, a magnetic circuitincluding said magnetic core and at least one rotor adapted uponrotation thereof to produce a changing magnetic flux in said magneticcore which varies with time in a saw-tooth manner.

References Cited in the file of this patent UNITED STATES PATENTS2,432,117 Morton Dec. 9, 1947

